The major objectives of this project are to identify the cell types that require TrkB-mediated signaling for normal development and synapse formation and function in the retina, cerebellum and neocortex. We will continue studies that have documented deficits in synaptic communication from photoreceptors to the inner retina in the absence of trkB signaling, attempting to determine which cells in the retina must express trkB for normal development and function of photoreceptors. We will also characterize the molecular changes that result in the observed synaptic deficit. Within the cerebellum, we will characterize the phenotypes caused by absence of TrkB signaling on development of Purkinje cell spines and expression of GABAergic proteins and development of synapses by Golgi interneurons. Again, we will identify the cells where absence of trkB results in these phenotypes and will attempt to identify proteins and mRNAs altered by absence of TrkB signaling that are candidates to explain these phenotypes. We will characterize further the progressive deficit observed in the cortex of mice in which TrkB has been eliminated from the majority of cortical pyramidal cells. We will determine whether it becomes more and more progressive as animals age, determine whether there are additional secondary effects on neurons not targeted directly, and will try to identify proteins and mRNAs known to be involved in dendrite formation that depend upon TrkB signaling for normal expression. Receptor tyrosine kinase signaling obviously mediates many diverse actions in the central nervous system. It is hoped that studies on this one important tyrosine kinase will result in conclusions applicable to understanding the roles of other tyrosine kinases in addition to the Trk receptors in regulating neuronal development, function, and aging in the central nervous system in both health and disease.